1. Field of the Invention
This invention relates generally to amplitude demodulators and in particular to a substantially ripple free pulse sample amplitude demodulator for use with sensitive equipment such as high gain amplifiers or rate detectors, which by their normal operation tend to aggravate any noise or ripple on the demodulated signal. Although the applications for this invention are many, it is particularly suited for use in high performance servosystems such as autopilot control surface servosystems.
2. Description of the Prior Art
The simplest amplitude modulation detector or demodulator is a diode rectifier. The ideal diode detector passes current in only one direction and will essentially follow the envelope of an amplitude modulated waveform. Although the simple circuit is quite effective in many applications, it requires heavy filtering to control ripple and is not useful in sensitive servosystems which require virtually ripple free demodulation, but are intolerant of heavy filtering. Heavy filtering using large capacitors, for example, is not advised in servo applications because large capacitors cause large time delays in circuit response which deteriorate system performance. To further complicate matters, many servosystems must respond to the velocity as well as the position of the system being controlled. In an autopilot system, such as the one shown in U.S. Pat. No. 3,848,833, issued to Applicants' assignee, for instance, the movement of the control surface, rudder, elevator, aileron, is closely servo controlled with respect to position and rate of change in position. The position information is taken from a detector such as a linear voltage displacement transducer or LVDT, which produces an a.c. signal and must be demodulated before use. The velocity information may be produced by a tachometer or it can be derived by differentiating the demodulated position signal. It is characteristic of differentiating or rate taking circuits that when a signal is processed therein, what was tolerable ripple at the input becomes greatly distorted and amplified at the output. For instance, a substantially d.c. signal having generally triangle shaped noise impulses thereon, becomes a d.c. signal with high amplitude rectangle noise spikes when differentiated. Thus, it is critical that ripple be eliminated from the demodulated position signal before it is differentiated and naturally, due to the attendant phase error, large filter capacitors must be avoided.
Most autopilot servosystems employ a pulse sample demodulator which, in the ideal case, takes a sample of the position signal waveform at precisely timed intervals coinciding with the waveform peak voltage. The sampled voltage is held in a holding circuit between peaks thus producing a true reading of the waveforms peak amplitude voltage. But no demodulator is ideal and because sampling times cannot be infintely short, the demodulated signal still contains a trace of the modulated position signal, for reasons that will be explained below. Thus, even with pulse sample demodulators of the conventional type, filter capacitors are required to sufficiently eliminate unwanted ripple. The present invention virtually eliminates this unwanted ripple at a low cost, using a nominal number of components, and without the necessity of filter networks which often require more components and printed circuit board space than does the invention.